1. Field of the Invention
The present invention relates to a sound field processor, and more particularly to a sound filed expanding apparatus that expands the sound field of a two-channel stereophonic sound.
2. Description of Related Art
In a conventional two-channel stereophonic apparatus, sound distribution that is generated by right and left speakers of the apparatus is limited within a range between the right loudspeaker and the left loudspeaker. A sound field expanding apparatus is used to expand the sound distribution to areas outside the speakers. In expanding the sound field of a two-channel stereophonic sound, an amplitude/phase characteristic changing circuit is used to add a specified amplitude characteristic and a phase characteristic to a signal on each of the two channels. Then, the two signals are added to each other.
However, when a sound field of original sound is merely expanded without any acoustic correction, reproduced sound is substantially, acoustically changed from the original sound. In this case, an acoustic correction is required to correctly reproduce the original sound. For example, an acoustic correction may be performed by a tone control method. However, the tone control method generally does not provide a satisfactory result. In order to correctly reproduce the original sound, influences resulting from an amplitude factor x(f) and a phase factor .theta. (f) that are added by the amplitude/phase characteristic changing circuit for expanding the sound field need to be eliminated from the signals with an appropriate method. For example, such influences may be removed with or without an inverse characteristic adder circuit that adds an inverse characteristic to the signals.
FIG. 5 shows an example of a sound field expanding apparatus that removes influences of amplitude and phase factors added by an amplitude/phase characteristic changing circuit without using an inverse characteristic adder circuit (see Japanese patent publication HEI 3-80400).
As shown in FIG. 5, amplitude/phase converter circuits 41a and 41b are provided for two inputs of left and right channels. The amplitude/phase converter circuits 41a and 41b include, respectively, inversion amplifiers 42a and 42b and amplitude/phase characteristic changing circuits 43a and 43b that change amplitude/phase characteristics of outputs from the amplitude/phase characteristic changing circuits 43a and 43b. Outputs from the inversion amplifiers 42a and 42b and outputs from the amplitude/phase characteristic changing circuits 43a and 43b are added to input signals of the respective channels by adders 46a and 46b, respectively. Outputs from the adders 46a and 46b are fed back as inputs to the respective inversion amplifiers 42a and 42b.
An output from the amplitude/phase characteristic changing circuit 43a of the amplitude/phase converter circuit 41a is added to an output from the inversion amplifier 42b of the other amplitude/phase converter circuit 41b by an adder circuit 44b, and is outputted through a tone control circuit 45b. In a similar manner, an output from the amplitude/phase characteristic changing circuit 43b of the amplitude/phase converter circuit 41b is added to an output from the inversion amplifier 42a of the other amplitude/phase converter circuit 41a by an adder circuit 44a, and is outputted through a tone control circuit 45a.
As shown in FIG. 6, each of the tone control circuits 45a and 45b shown in FIG. 5 is formed from, for example, an operation amplifier OP, fixed resistances R1, R2 and R3, variable resistances VR1 and VR2, and capacitors C1 and C2. The capacitor C1 (which is a low-frequency cut capacitor) is connected in parallel with the variable resistance VR1 that adjusts a gain in a low-frequency band. The capacitor C1 and the variable resistance VR1 define a section that controls the low-frequency band. In other words, the capacitor C1 and the variable resistance VR1 define a section that functions as a bass control section. The capacitor C2 and the variable resistance VR2 through which an output signal from the operation amplifier OP is fed back define a section that controls a high-frequency band. The capacitor C2 and the variable resistance VR2 define a section that functions as a treble control section.
In the above-described conventional sound field expanding apparatus, the tone control circuits 45a and 45b require capacitors having a large capacitance. In particular, the capacitor C1 in the bass control section requires a substantially large capacitance that amounts to several .mu. F. As a consequence, it is difficult to implement such a sound field expanding apparatus in an LSI because it is difficult to internally mount these capacitors in an LSI, and therefore external devices have to be attached to an LSI.